Aluminum nitride body and method for forming said body utilizing a vitreous sintering additive

ABSTRACT

An unsintered aluminum nitride body including: 
     (a) 1 to 5 weight percent of a vitreous solid of boria, alumina, and calcia in the proportions of (1) boria between 3 and 25 weight percent, (2) alumina between 10 and 50 weight percent, and (3) calcia between 40 and 80 weight percent; and 
     (b) aluminum nitride powder as the balance of the aluminum nitride body. 
     The invention further relates to a method of forming the unsintered aluminum nitride body and then sintering it at a temperature between 1550 and 1650 degrees Centrigrade so as to form a dense, thermally conductive aluminum nitride body.

This application is a divisional of Ser. No. 08/173,293 filed Dec. 22,1993, and now U.S. Pat. No. 5,482,903.

BACKGROUND OF THE INVENTION

This invention relates to a method for the production of an aluminumnitride sintered body and, more particularly, relates to an unsinteredaluminum nitride body and a method for forming such body followed bysintering so as to produce an aluminum nitride sintered body having ahigh density and satisfactory thermal conductivity by sintering at atemperature lower than that in conventional techniques.

Aluminum nitride has been of interest recently for electronic packagingapplications because of its high thermal conductivity, thermal expansionmatching with silicon, low dielectric constant (8.5) and high electricalresistivity.

Production of an aluminum nitride sintered body having excellent thermalconductivity requires forming of aluminum nitride powder followed bysintering to achieve a dense body.

Since aluminum nitride by itself is difficult to sinter underatmospheric pressure, pressureless sintering of aluminum nitride hasconventionally been carried out with the aid of additive sintering aids.

For example, Takeshima et al. Japanese Kokai J04-154,670 disclosesdense, sintered aluminum nitride bodies which have been achieved by theuse of alumina and calcia additions.

Most of the additives proposed previously are highly refractorymaterials, and remain in the solid state during the early stage ofsintering. During the course of sintering, some of them eventually forma liquid phase by reacting with any aluminum oxide present, either byintentional addition or with the aluminum oxide impurity in aluminumnitride. The liquid phase thus formed has been reported to aiddensification of aluminum nitride. However, because of the refractorynature of these additives, the temperature required for sinteringaluminum nitride has been exceptionally high (1800-2000 degreesCentigrade) compared to the sintering temperature of 1500-1600 degreesCentigrade for alumina.

Others have proposed lower melting additives such as boria in additionto calcia and alumina. Nakano et al. Japanese Kokai J02-275,769discloses adding additions of aluminum, calcia and boria to aluminumnitride powder, followed by sintering at 1400-2000 degrees Centigrade.However, to achieve a fully dense body having a thermal conductivity of192 W/m-K, the compositions were sintered at 1800 degrees Centigrade for4 hours. Sawamura, et al., Japanese Kokai J62-176,961 disclosesadditions or mixtures of alumina, calcia and boria (as well as others)to aluminum nitride to achieve a sintered body. In one example, amixture of 7 weight percent 3Al₂ O₃ ·5CaO and 1 weight percent B₂ O₃ wasadded to aluminum nitride and sintered for 2 hours. The resultingthermal conductivity was 70 W/m-K. Boria, by itself, however, melts atabout 450 degrees Centigrade which presents difficulties in electronicpackaging applications. For example, it is necessary to removesubstantially all residual carbon from substrates that are used inelectronic applications. The low melting boria hinders this so-calledbinder burnoff process.

Finally, others have proposed the addition of sintering aids in the formof vitreous materials. Saito et al. Japanese Kokai J03-218,977 disclosesthe addition of 0.1-10 weight percent of a glass powder sintering aid tothe aluminum nitride powder prior to sintering. The glass powderconsists of 0-38 mole % alumina, 30-80 mole % boria and 20-56 mole %calcia. In weight percent, it is 0-28 weight % alumina, 27-77 weight %boria and 23-64 weight % calcia. The aluminum nitride body is sinteredat a temperature greater than 1650 degrees Centigrade which isundesirably high. The resulting aluminum nitride samples have a maximumthermal conductivity of 110 W/m-K which, while better than alumina, isconsiderably less than pure aluminum nitride. Further, the majority ofsamples, however, had a thermal conductivity of 100 W/m-K or less.

Accordingly, the present inventors have proposed adding acalcia-alumina-boria glass to aluminum nitride powder in a way, and inan amount, sufficient to obtain a dense, highly thermally conductivebody.

It is thus a purpose of the present invention to produce an aluminumnitride body that is dense and highly thermally conductive.

It is another purpose of the present invention to produce an aluminumnitride body by a sintering process at a lower sintering temperaturethan has heretofore been possible which will allow the production of thealuminum nitride body at a reduced cost.

These and other purposes of the present invention will become moreapparent after referring to the following detailed description of theinvention.

BRIEF SUMMARY OF THE INVENTION

The purposes of the invention have been achieved by providing, accordingto one aspect of the invention, an unsintered aluminum nitride bodycomprising:

(a) 1 to 5 weight percent of a vitreous solid powder of boria, alumina,and calcia in the proportions of (1) boria between 3 and 25 weightpercent, (2) alumina between 10 and 50 weight percent, and (3) calciabetween 40 and 80 weight percent; and

(b) aluminum nitride powder as the balance of the aluminum nitride body.

According to a second aspect of the invention, there is provided amethod of forming an aluminum nitride body comprising the steps of:

(a) preparing a mixture of boria, alumina, and calcia in the proportionsof (1) boria between 3 and 25 weight percent, (2) alumina between 10 and50 weight percent, and (3) calcia between 40 and 80 weight percent;

(b) melting the mixture to form a homogeneous liquid;

(c) quenching the liquid, thereby attaining a homogeneous vitreoussolid;

(d) pulverizing the vitreous solid to obtain a predetermined particlesize;

(e) adding the pulverized vitreous solid to aluminum nitride powder, inthe proportions of (1) pulverized vitreous solid between 1 and 5 weightpercent, and (2) aluminum nitride powder the balance; and

(f) sintering the resulting mixture of pulverized vitreous solid andaluminum nitride powder at a temperature sufficient to causedensification of the mixture into a dense aluminum nitride body.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to the production of an aluminum nitride bodyhaving a high density and good thermal conductivity. More specifically,this invention relates to the production of an aluminum nitride body ata sintering temperature that is lower than is currently practiced.According to the invention, the aluminum nitride body may be producedby:

(a) preparing a mixture of boria, alumina, and calcia in the proportionsof (1) boria between 3 and 25 weight percent, (2) alumina between 10 and50 weight percent, and (3) calcia between 40 and 80 weight percent;

(b) melting the mixture to form a homogeneous liquid;

(c) quenching the liquid, thereby attaining a homogeneous vitreoussolid;

(d) pulverizing the vitreous solid to obtain a predetermined particlesize;

(e) adding the pulverized vitreous solid to aluminum nitride powder, inthe proportions of (1) pulverized vitreous solid between 1 and 5 weightpercent, and (2) aluminum nitride powder the balance; and

(f) sintering the resulting mixture of pulverized vitreous solid andaluminum nitride powder at a temperature sufficient to causedensification of the mixture into a dense aluminum nitride body.

Aluminum nitride bodies produced according to this process areparticularly suitable for co-fired multilayer electronic packages wherea metallic paste is sintered along with the aluminum nitride body. Thealuminum nitride bodies may be produced by slurry casting, hot pressingor some other method well known in the art. The forming process and thedesired density and thermal conductivity will dictate the composition ofthe aluminum nitride body as will become apparent hereafter.

The process begins with a mixture of boria (B₂ O₃), alumina (Al₂ O₃) andcalcia (CaO). The proportions of each of the components, in weightpercent, are: 3-25% boria, 10-50% alumina and 40-80% calcia. Thismixture is placed in a suitable crucible in a furnace and then heated toabout 1450 degrees Centigrade to cause the components to melthomogeneously. Thereafter, the melt is rapidly quenched, such as bysplat quenching between steel plates, or by roller quenching betweencooled drums, so that the melt, once cooled to room temperature, forms asolid of clear vitreous character.

Upon quenching, the vitreous solid will be in the form of glass ribbonswhich are unsuitable for the present invention. The vitreous solid mustthen be pulverized or comminuted so as to obtain the desired particlesize. A preferred particle size for the present invention is about 3 to5 microns. The particles of the vitreous solid are then dried byconventional means and stored for the next step of the process.

Now, the aluminum nitride body may be formed. An amount of 1 to 5 weightpercent of the pulverized vitreous solid is added to aluminum nitridepowder. The aluminum nitride powder should have a particle size of about1-1.5 microns. Aluminum nitride powder is available from a number ofsources such as Tokuyama Soda (grades F and H) and Dow Chemical (grades44 and 48). For the most preferred embodiment of the invention, thepresent inventors prefer the Tokuyama Soda, grade F, aluminum nitridepowder because of its low oxygen content and more uniform particledistribution which, the inventors have found, leads to greaterdensification and higher thermal conductivity. To this mixture may beadded a binder material which can assist in holding the body together inthe green state prior to sintering. Suitable binders are ethylcellulose, polyvinyl butyral and polymethyl methacrylate (PMMA), all ofwhich are conventional and well known. The amount of binder materialwill depend on the actual forming operation that is utilized to form thealuminum nitride body.

If, for example, the aluminum nitride body is to be formed by hotpressing, only about 5 weight percent binder material need be added tothe mixture of aluminum nitride and pulverized vitreous solid.

A slurry casting process can also be used with this invention but it isa more complicated process. For slurry casting, a mixture is madeconsisting of ceramic materials (aluminum nitride powder plus thepulverized vitreous solid), binder, solvent and minor conventionalconstituents such as plasticizers and anti-oxidants. The binder may beabout 5 to 15 weight percent while the solvent amounts to about 20 to 45weight percent, the remainder being the ceramic materials. The slurry iscast onto a carrier sheet, usually of a polymeric material whichconventionally may be Mylar. Upon drying, the carrier sheet is removedand a tape of the product is produced. The tape is blanked into thedesired endshape. One desired endshape is a greensheet for fabricatingmultilayer ceramic packages.

Multilayer ceramic packages may be fabricated by the following process.A series of greensheets are punched to form "vias" and then a metallicpaste is screened onto the greensheets, to form conductive lines, andinto the vias to form conductive pathways between the different layersof greensheets. For aluminum nitride products, the preferred metallicpastes contain molybdenum, tungsten, or mixtures of molybdenum andtungsten. The greensheets are then stacked, laminated and sintered toobtain a multilayer ceramic package. In use, at least one semiconductordevice is mounted on the multilayer ceramic package. The multilayerceramic package is a preferred use of the present invention.

The aluminum nitride body may be sintered in a conventional furnace solong as there is a protective atmosphere. A preferred atmosphere isforming gas which is a mixture of nitrogen and hydrogen gases. A typicalsintering schedule can be undertaken as follows. The unsintered aluminumnitride bodies are inserted into a sintering furnace. A protectiveatmosphere of dry forming gas (N₂ +10-20% H₂) is used throughout thesintering process. Over a period of about 5 hours, the temperature isramped up from room temperature to about 600 degrees Centigrade topyrolyze the binder. Then, over a period of about 8 hours, thetemperature is ramped up to the sintering temperature of about 1550-1650degrees Centigrade and held at the sintering temperature for about 5hours. Then, over a period of about 5 hours, the temperature is rampeddown to room temperature.

As noted above, the proportions of the components of the vitreous solid,in weight percent, are 3-25% boria, 10-50% alumina and 40-80% calcia. Ithas been found, however, that excessive amounts of calcia and boria cancause gelling of the casting slurry making it, for all intents andpurposes, uncastable. Therefore, for slurry casting, it is preferredthat the proportions of the components of the vitreous solid, in weightpercent, are 5-20% boria, 20-45% alumina and 45-65% calcia. For greatestdensity and thermal conductivity, it is most preferred that theproportions of the components of the vitreous solid, in weight percent,are 5-15% boria, 30-40% alumina and 45-55% calcia.

Generally speaking, if there is too much boria, the glass is tooreactive with the ambient while if there is too little boria (e.g. belowabout 3 weight percent), there is little advantageous effect. Aboveabout 20 weight percent boria, there is undesirable gelling of thecasting slurry. With respect to alumina, too much alumina reducesthermal conductivity while if alumina is reduced too much, the glassbecomes chemically less durable and therefore it is not practical.Calcia also needs to be limited since inordinately high amounts causegelling of the casting slurry.

There are three important features of the present invention. The firstis that the sintering aid is introduced as a vitreous additive ratherthan as separate components which form a liquid phase in-situ. In thisway, more uniform sintering is obtained in that there is a definitetemperature at which the vitreous additive will become liquid. For thepresent boria-alumina-calcia system, viscous flow begins at around 800degrees Centigrade with the vitreous additive actually becoming liquidat around 1270 degrees Centigrade. The sintering temperature is between1550 and 1650 degrees Centigrade. The formation of the liquid phase inthe system at such an early stage of sintering helps densification ofaluminum nitride bodies. In the ternary boria-alumina-calcia system, theglass forming range, where one can obtain more thermally stable glasses,has been known for the composition with more than 30 weight % of boriaas reported by Owen (A. E. Owen, Phys. Chem. Glasses, 2[3] pp. 87-98,1961). However, it has been found in the present invention that (1) onecan form vitreous solid for the composition far outside of the reportedglass forming range, with much less boria incorporation, by rapidquenching; (2) viscous flow of such less boria glasses occurs at 800degrees Centigrade or higher which is higher then the temperature rangewhere pyrolysis of most binders occurs; (3) such less boria glassescrystallize at about 900 degrees Centigrade to limit the viscous flow sothat the calcined body remains porous for ease of any additionalreaction for binder residues; (4) crystallized phases of such less boriaglasses melt at about 1250 degrees Centigrade which is 100 degreesCentigrade lower then the eutectic temperature of calcia-alumina systemwithout boria.

The second important feature is the use of a fugitive constituent. Thefugitive constituent, boria, lowers the liquidus of the second phase toenhance densification, but will eventually leave the system byevaporation, thereby reducing the volume fraction of the second phaseafter sintering, and thus minimizing possible degradation of the thermalconductivity of the sintered aluminum nitride body by the sintering aid.To control the evaporation of the fugitive constituent, it is necessaryto encapsulate the aluminum nitride body, such as with molybdenumbaffles. It is believed that aluminum nitride, boron nitride or graphitebaffles would work as well. Such encapsulation should not be too tightbut, without adequate encapsulation, any non-uniformity in the boriacontent in the sintering aluminum nitride bodies will cause seriouswarping and non-uniform densification.

The last important feature of the present invention is that the carbonresidue resulting from pyrolysis of the binder material mayadvantageously be used to effect in-situ carbothermal reduction andnitridation of oxides. The residual carbon cannot be burned off due tothe non-oxidizing atmosphere; however, the residual carbon is utilizedfor carbothermal reduction and nitridation, which are the same processesused to produce aluminum nitride, to minimize the oxygen content in thesintered aluminum nitride body. In practice, a forming gas atmosphere(e.g., N₂ +20% H₂) is used. Such a reducing atmosphere would be favoredfor carbothermal nitridation, seen from the following chemicalreactions:

    Al.sub.2 O.sub.3 +N.sub.2 +3C=2AlN+3CO↑

    3CaO+N.sub.2 +3C=Ca.sub.3 N.sub.2 ↑+3CO↑

The first reaction is well known while the second one, while feasible,is not well established. The end result, however, is reduction of theoxides present, thereby increasing the thermal conductivity.

The advantages of the present invention will become more apparent afterreferring to the following examples.

EXAMPLES

A number of samples were fabricated in the following manner. Severaldifferent glass compositions were made wherein the relative proportionsof calcia, boria and alumina were varied. The glass particles werepulverized by ball milling with methanol (zirconia media and aluminajar) for 14-15 hours and then poured into a shallow tray for drying. Thecrushed vitreous particles were then added to aluminum nitride particlesand 10% of polyvinyl butyryl binder solution and ballmilled in analumina jar (nylon or alumina media) for 14 hours to prepare a uniformslurry. The aluminum nitride powder was either Dow Chemical, grade 44 or48, or Tokuyama Soda, grade F. The ballmilled slurry was tape cast bythe doctor-blade method to form 6 mils thick tape. The tape was thenblanked to square sheets, 20 of which were stacked, laminated andsintered at 1580 degrees Centigrade for 10 hours (except for one sample(^(**)) which was sintered for 24 hours) and then cooled to roomtemperature. The results are tabulated in Table I below. The percentdensification was determined by the size of the sintered sample againstthe theoretical value and the thermal conductivity was determined by thelaser flash method.

                  TABLE I                                                         ______________________________________                                                                           Thermal                                    CaO/Al.sub.2 O.sub.3 /                                                                 AlN.sup.+        %        Conductivity                               B.sub.2 O.sub.3 Ratio                                                                  Type    % Glass  Densification                                                                          (W/m-K)                                    ______________________________________                                        70/20/10 D48     3.5      Gelled                                              60/30/10 D48     3.5      Gelled                                              50/40/10 T-F     3.1      97.5      148**                                              D44     4.5      94.3     101.5                                               D44     3.5      95.4     100.7                                               D48     3.5      94.3     --                                                  T-F     3.5      95.9     110.4                                               T-F     5.0      97.2                                                50/30/20 D48     2.0      85.1                                                         T-F     3.0      95.6     123.8                                               T-F     5.0      94.5     120.0                                      ______________________________________                                         .sup.+ For type of AlN used, D48 is DowChemical grade 48, D44 is              DowChemical grade 44, TF is Tokuyama Soda grade F.                            **Sintered for 24 hours.                                                 

As can be seen from Table I, the samples with 60 and 70 weight percentcalcia gelled. Since the inventors were most interested in castableslurries, these samples were not further investigated. If the bestsamples are taken (i.e., those made with Tokuyana soda, Grade F powder),a thermal conductivity of 110-120 W/m-K can be obtained for densealuminum nitride bodies at a relatively low sintering temperature of1580 degrees Centigrade. The thermal conductivity can be furtherenhanced to 148 W/m-K by extending the sintering time to 24 hours. Thus,the advantages of the present invention are apparent, namely, densealuminum nitride bodies with a satisfactory thermal conductivity as aresult of pressureless sintering at a relatively low temperature. Withoptimization of the aluminum nitride powder and sintering time, it isexpected that the thermal conductivity could be increased even further.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of this invention beyond thoseembodiments specifically described here may be made without departingfrom the spirit of the invention. Accordingly, such modifications areconsidered within the scope of the invention as limited solely by theappended claims.

What is claimed is:
 1. A method of forming an aluminum nitride bodycomprising the steps of:(a) preparing a mixture of boria, alumina, andcalcia in the proportions of (1) boria between 3 and 25 weight percent,(2) alumina between 10 and 50 weight percent, and (3) calcia between 40and 80 weight percent; (b) melting the mixture to form a homogeneousliquid; (c) quenching the liquid, thereby attaining a homogeneousvitreous solid; (d) pulverizing the vitreous solid to obtain apredetermined particle size; (e) adding the pulverized vitreous solid toaluminum nitride powder, in the proportions of (1) pulverized vitreoussolid between 1 and 5 weight percent, and (2) aluminum nitride powderthe balance; and (f) sintering the resulting mixture of pulverizedvitreous solid and aluminum nitride powder at a temperature sufficientto cause densification of the mixture into a dense aluminum nitridebody.
 2. The method of claim 1 wherein the sintering temperature is inthe range of 1550 to 1650 degrees Centigrade.
 3. The method of claim 1further comprising the step between steps (e) and (f) of combining abinder material with the pulverized vitreous solid and aluminum nitridepowder to form a slurry and casting the slurry into greensheets.
 4. Themethod of claim 1 wherein, in step (a), the amount of boria present isbetween 5 and 20 weight percent, the amount of alumina present isbetween 20 and 45 weight percent, and the amount of calcia present isbetween 45 and 65 weight percent.
 5. The method of claim 4 wherein theamount of boria present is between 5 and 15 weight percent, the amountof alumina present is between 30 and 40 weight percent, and the amountof calcia present is between 45 and 55 weight percent.
 6. The method ofclaim 3 wherein there are a plurality of greensheets and furthercomprising the step of depositing a metallic composition on at least oneof the greensheets.
 7. The method of claim 6 wherein the metalliccomposition comprises molybdenum, tungsten or mixtures thereof.